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POpreventing "ld -wrap " circular references
 

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  1. POpreventing "ld -wrap " circular references
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    <p>I am using GNU ld's "-wrap" option to intercept calls in applications, but have ran into a scenario where the code implementing the wrapper indirectly calls the wrapped function, creating a circular reference.</p> <h2>Example</h2> <p>The goal is to wrap read calls that occur in Program Foo. This code can be recompiled/relinked, but not modified.</p> <h3>Program Foo</h3> <pre><code>main() { ... read(fd, buf, size); ... } </code></pre> <p>The wrapper here will intercept calls to libc's read in Program Foo when using "-wrap read".</p> <h3>Wrapper</h3> <pre><code>extern int __real_read(...); int __wrap_read(...) { bar(); __real_read(...); } </code></pre> <p>However, Library Bar, called from the wrapper needs to use libc's read() function without going through the wrapper (thus causing the circular dependency).</p> <h3>Library Bar</h3> <pre><code>void bar(void) { read(fd, buf, size) } </code></pre> <p>Changing all wrapped routines in Library Bar to use __real_read() is not an option because the level of indirections that exist inside additional calls to external libraries in Library Bar are arbitrary.</p> <h2>Avoiding Flags</h2> <p>One way to solve this is to use per-thread flags to prevent wrapped reads originating from Library Bar reentering the library. Although I would prefer to not use this solution, I'm also open to suggestions on how to implement this with minimal code change in the wrapper and bar libraries.</p> <h2>Ideal Solution</h2> <p>??? That is why I'm asking the question :)</p> <p>Thanks... -n</p>
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    1. USNoah Watkins
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      1. USJoseph A. Levin
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    1. COCan you direct the linker to only apply the -wrap to certain object files? Maybe a linker script can get you there.
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      1. USnmichaels
    2. COThis is unlikely. Consider the case where bar() called a function in another object file (and so forth) which in turn called read(). Those levels of indirection would quickly grow large and tracking it all would error prone.
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      1. USNoah Watkins
 

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